The need to store and quickly access large amounts of data is becoming increasingly important as memory-intensive applications evolve. Additionally, energy-constrained systems (e.g., sensor networks) demand low-energy, non-volatile memory. There is thus a need for truly “universal” memory device that satisfies all these requirements. Magnetic hard disk drives are non-volatile, extremely dense, and low-cost, but operate at very slow speed. Flash memory can also be non-volatile and accessed and written to much faster than a hard drive, but still considerably slower the on-chip cache. Cache memory (SRAM) and main memory (DRAM) are typically fast, require a stable power supply, and can have higher cost-per-bit than flash memory.
In the past few years, a non-volatile, high-density, low-energy serial magnetic memory known as “racetrack” has received much attention as a potential candidate to replace flash or DRAM memories. Racetrack memory involves shifting magnetic domain walls, regions of opposing magnetization in a magnetic material, between stable domain wall positions (notches) along a track. The domain wall shifting mechanism is current-driven, traditionally spin transfer torque (STT), where a spin-polarized input current imparts a torque on local magnetic moments causing them to align in the direction of the electron spin. Stored data bits of the racetrack are read out serially via a magnetic sensor (e.g., magnetic tunnel junction) in proximity to the track somewhere along the shift register.
Although elegant in its simplicity, racetrack memory has fundamental issues of control that make implementing it a major challenge. Specifically, causing a movement of a multitude of domain walls by a single bit position (e.g., no more and no less) with a single current pulse down the track can be difficult. Variability in domain wall pinning potential along the track, the inherent stochasticity of domain wall motion, and the inability to margin the current pulse make data bit errors likely.
Therefore, a need exists for a non-volatile, high-density, low-energy serial magnetic memory with, for example, control at a level of single bit positions.